Production of cyclo-octa-tetraene



United States Patent 2,912,472 PRODUCTION OF CYCLO-OCTA-TETRAENE Walter Reppe, Heidelberg,

Karl Dury, Ludwigshafen (Rhine),

and Franz Reicheneder. and and Albert Zieger,

Heidelberg, Germany, assignors to Badische Anilin- & Soda-Fabrik Aktiengesellschatt, Ludwigshafen (Rhine), Germany This invention relates to improvements in the production of cyclo-octa-tetraene by the cyclizing polymerization of acetylene.

It has already been proposed to prepare cyclo-octatetraene by cyclizing polymerization of acetylene, the catalysts used being especially halides, cyanides, rhodanides or enolates of nickel, and anhydrous tetrahydrofurane being used for example as a solvent. The speed of formation of cyclo-octa-tetraene depends on the temperature and the amount of acetylene present in the reaction zone. Whereas the optimum temperature for the formation of cyclo-octa-tetraene could easily be found, it has been impossible so far to use satisfactorily high acetylene concentrations in the reaction zone for instance by applying high pressures. The difficulties are in the nature of acetylene, in particular its readiness to decompose. The practice has therefore been to use pure aotylene under very low pressures, or acetylene diluted with inert gases, such as nitrogen, under moderately elevated pressures. These conditions have hitherto been found to be in the explosion range of the acetylene, and indeed not only in the gas phase, but also often in the liquid phase of the reaction mixture. This necessitated expensive safety precautions.

The US. patent specification 2,613,231 describes a process which is carried out at 7.5 to 45 atmospheres total pressure, the partial pressure of the acetylene not exceeding to 35 atmospheres to prevent detonations. At a total pressure of 45 atmospheres and 22.5 atmospheres partial pressure of the acetylene, i.e. when using a gas mixture containing 50% of acetylene, the process is doubtless carried out in the explosion range of acetylene. At a total pressure of 45 atmospheres the upper limit of the partial pressure of the acetylene in this process is reached with an acetylene content in the gas mixture of about 80%. This is definitely in the detonation range.

Copenhaver et al., in Acetylene and Carbon Monoxide Chemistry, 1949, teach that acetylene is liable to decompose at a total pressure of 6 atmospheres, if the nitrogen content in the acetylene-nitrogen mixture falls below 49% which corresponds to a partial pressure of the acetylene of about 3 atmospheres. Investigations covering a pressure range up to 32 atmospheres showed that detonations could be avoided and the acetylene decomposition controlled, but there was no possibility to work outside the explosion range at increased pressures. Within the pressure range known from the investigations to be outside I the explosion range commercial-scale production of cycloocta-tetraene with saisfactory space-time yields was not feasible.

' We have now found that cyclo-octa-tetraene can be produced by cyclizing polymerization of acetylene under increased pressure in a technically advantageous way by carrying out the reaction with an inert gas, such as nitrogen, which is under a total pressure of more than 75 atmospheres absolute but below the decomposition limiting pressure of the gas mixture and the concentration of the acetylene being below the ignition limiting concentration.

a mixture of acetylene and By the method according to our invention acetylene can be handled safety within a pressure range of from to 400 atmospheres, if the acetylene content in the gas mixture is adjusted in accordance with the decomposition curve shown .in' the figure. From the diagram it can be seen that the acetylene synthesis can be carried out, in the overall pressure range, at optimum temperatures and outside the explosion zone, if the acetylene content in the mixture of acetylene and inert gas is reduced from about 23% to 20%. Gas mixtures containing less than 10% of acetylene are disadvantageous.

The safety precautions necessary with the prior art methods, e.g. designing the apparatus for 10 times the reaction pressure, or dividing up the free gas spaces, for instance by providing the separators with filler bodies, or using bundles of: tubes instead of single tubes with large diameters, are now dispensable. Of course, it is also possible to work below some 75 atmospheres with a mixture of acetylene and an inert gas diluted according to the data given in Figure 1; however, the partial pressures obtainable for acetylene at pressures below 75 atmospheres are insufficient for carrying out the synthesis on a commercial scale. Whereas space-time yields of up to 1.14, i.e. 1.14 kg. of cyclo-octa-tetraene to 1 liter of catalyst space per day, can be obtained at pressures above atmospheres, i.e. partial pressures of the acetylene of about 20 atmospheres, the space-time yield falls below the economic minimum of 0.5 at total pressures below 75 atmospheres. If the process is carried out in the conventional explosion-free pressure range of up to 32 atmospheres, e.g. at 20 atmospheres total pressure and 7 atmospheres partial pressure of the acetylene, cyclo-octa-tetraene is formed only in such an amount that the space-time yields are below 0.2.

The process can be carried out at elevated temperatures, for instanceat temperatures of 75 to C., preferably 85 to 125 C. It is advantageous to work at total pressures of to 250 atmospheres.

As catalysts those in common use for the cyclo-octatetraene synthesis, e.g. halides and pseudohalides, e.g. cyanides, thiocyanates, or enolates of the bivalent nickel, especially nickel acetyl acetonate or the nickel enolate of 'ethylacetoacetate are used. Other compounds, e.g. aluminum organic compounds such as aluminum enolate of alkyl acetoacetonate, may be added to these catalysts as activators. The catalysts are usually introduced into the reaction zone, dissolved or suspended in the solvents, in quantities of 0.05% to 4.0%, preferably 0.2% to 0.9%, based on the solvent used.

The decomposition limiting pressure of mixturesof acetylene and inert gases, i.e. gases which do not react with acetylene and cyclo-octa-tetraene, such as nitrogen or rare gases, is dependent on the composition of the gas mixture.

The accompanying drawing is a diagram showing the course of the decomposition limiting pressure of a mixture of acetylene and nitrogen in dependence upon its acetylene content. The ordinates are the acetylene content of the gas mixture in percent by volume, the remainder being nitrogen. The abscissae are the total pressure in absolute atmospheres on a logarithmic scale. represents the measurement points at which no decomposition occurs upon ignition and X represents meas urement points at which decomposition occurs. The mixing ratios and pressures which are to be maintained according to the present invention are therefore below the curve in the drawing.

The decomposition limiting pressures have been determined in a stainless steel tube 1 meter long and 90 millimeters in internal width in the way described in the monograph of W. Reppe, Chemie und Technik der 3 Acetylen-Druckreaktionen, Verlag Chemie, Weinheim (1951), pages 1 to 19, by ignition by means of a platinum wire heated to fusion.

From the diagram it may beseen that for example at total pressures of 100 or 200 atmospheres and more gas mixtures which contain up to about 23 or 20 percent, respectively, or acetylene can be used without danger.

The upper limit of the acetylene pressure to beused is dependent on the solubility of acetylene in the solvent used or in the reaction mixture and on the limiting ignition concentration of the dissolved acetylene at the reaction temperature in question.

The following table summarizes the results of some ignition tests of solutions of acetylene in different solvents; the indicated acetylene concentrations show the upper limiting concentrations at which no ignition or detonation have been observed at about 90 C.

Cubic centimeters of gaseous Therefore it is possible to Work without danger under a total pressure at which the given solubility of acetylene and consequently the ignition limit of the solution is not exceeded. In general the reaction medium, i.e. the diluent, may contain up to 160 cubic centimeters of acetylene per cubic centimeter of solvent.

The new process herein described renders it possible to prepare cyclo-octa-tetraene in apparatus such as is customary for working with non-explosive gases. It is thus not necessary to use vessels designed for pressures up to to times the working pressure and to avoid large free gas spaces by the provision of insertions, such as have hitherto been necessary. Moreover no explosion can take place even if the catalyst becomes dry for any reason.

Moreover the process according to this invention can be modified by first saturating the solvent, for example tetrahydrofurane free from catalyst in a separate apparatus at room temperature with acetylene or with a mixture of acetylene and an inert gas, such as nitrogen, by forcing in the same under such a pressure that the ignition limit of the liquid phase is still not reached, and only then pumping the resultant solution saturated with acetylene into the actual reaction vessel containing the catalyst or its solution. During this previous saturation the usual safety precautions for working with acetylene under pressure must be used, but the apparatus involved for that purpose is only a relatively small part of the whole apparatus, whereas in the actual reaction to cyclo-octatetraene such precautions are superfluous. This modified method of working has the advantage that compression costs are saved.

The process according to this invention, in either of its embodiments, thus represents an enrichment of the art by enhanced safety and by saving expensive safety precautions which had hitherto been thought to be indispensable.

The following examples will further illustrate this invention but the invention is not restricted to these examples.

Example 1 Into a 3 liter sump oven designed for a pressure of about 150 atmospheres there are pumped per hour 500 cubic centimeters of dry tetrahydrofurane containing about 0.5 to 1 percent of nickel acetyl-acetonate dissolved therein. Into the sump of the vessel there is led at the same time at about 100 atmospheres total pressure a mixture of about 22 percent by volume of acetylene and about 78 percent by volume of nitrogen. The temperature of the vessel is adjusted to about to C. The gas mixture is circulated at a pressure of about atmospheres by means of a circulatory gas pump. The waste gas is kept at an acetylene content of about 17 to 18 percent by volume by the supply of acetylene. The reaction product consists of about 10 percent cyclo-octa-tetraene solution in tetrahydrofurane which contains but a small amount of benzene and a solid black residue including small amounts of the catalyst which have not yet been consumed. The reaction mixture is worked up in the usual way by filtration and fractional distillation.

Example 2 In modification of the method of operation described in Example 1, the dry tetrahydrofurane containing about 0.5 to 1 percent of nickel acetyl acetonate is first pumped into a high pressure tube filled with filler bodies which is kept at about 20 C. under a pressure of about 7 atmospheres of acetylene. The solvent here saturated with acetylene is then pumped by means of a second pump into the reaction vessel the temperature of which is adjusted to about 85 to 95 C. A gas mixture of about 20 percent by volume of acetylene and about 80 percent of nitrogen is circulated at about 100 atmospheres through the reaction vessel. By working in this way it is possible to keep the acetylene content in the circulating gas at about 20 percent by volume with a smaller amount of fresh gas and only slight amounts of waste gas being branched off. The yields of cyclo-octa-tetraene are just as high as in Example 1.

Example 3 The synthesis is initiated as in Example 2 but the supply of tetrahydrofurane containing the nickel acetyl acetonate is increased to an amount of about 600 cubic centimeters per hour in all. After the initiation of the reaction, the total pressure of the circulating gas mixture is slowly increased up to about 200 atmospheres corresponding to the diminishing solubility of the acetylene in the reaction solution by reason of the products formed, the acetylene content in the circulating gas being adjusted to about 20 percent by volume.

The reaction product has a cyclo-octa-tetraene content of about 10 percent besides the byproducts mentioned in Example 1.

Example 4 Into a 24 liter sump oven designed for pressures up to 325 atmospheres there are pumped per hour 6.5 liters of dry benzene in which 0.6% of nickel acetyl acetonate has been dissolved. Before the solution is introduced into the sump oven it is saturated with pure acetylene in a high-pressure tube charged with filler bodies, at a pressure of 7 atmospheres and a temperature of 20 C. At the same time 3.5 liters of an 0.35 percent benzene solution of aluminum enolate of ethylacetoacetate pressed into the sump oven per hour by a gas mixture of 20 percent by volume of acetylene and 80 percent by volume of nitrogen at a pressure of 200 atmospheres, after the solution has previously been treated with the said gas mixture in a second high-pressure tube at 130 to 140 C. and 200 atmospheres pressure. The temperature of the Sump is kept between and C. and should not rise above C. After passing through the vessel part of the gas mixture which is introduced into the sump is cycled back by means of a gas pump. The rest of the gas mixture is discharged into the open. By introducing fresh gas the acetylene content of the gas phase in the sump oven is adjusted to 12 to 14 percent. The reaction product which is discharged at the bottom of the sump oven consists of a 10 percent solution of cyclo-octa-tetraene in benzene in which negligible parts of a nearly black residue are suspended. The reaction mixture is worked up by steam distillation and subsequent fractional distillation.

This application is a continuation-in-part of our application Serial No. 641,003, filed February 15, 1957 (now abandoned).

We claim:

1. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of an inert diluent and a bivalent nickel compound as a catalyst, the improvement which comprises carrying out the reaction with a mixture of acetylene and an inert gas at a total pressure of more than 75 atmospheres up to about 400 atmospheres, at a concentration of acetylene in the gas phase which is at least by volume but below a concentration falling-above the decomposition limiting pressure of the gas mixture and at a concentration of the acetylene in the liquid phase which is below the ignition limiting concentration of the liquid mixture.

2. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of an inert diluent and a bivalent nickel compound selected from the group consisting of bivalent nickel halides, pseudohalides, and enolates as a catalyst, the improvement which comprises carrying out the reaction with a mixture of acetylene and an inert gas at a total pressure between 75 atmospheres and 400 atmospheres, at a concentration of acetylene in the gas phase which is at least 10% by volume but below a concentration falling above the decomposition limiting pressure of the gas mixture and at a concentration of the acetylene in the liquid phase which is below the ignition limiting concentration of the liquid mixture.

3. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of an inert diluent and a bivalent nickel compound v as a catalyst, the improvement which comprises carrying out the reaction with a mixture of acetyleneand an inert gas at a total pressure of 150 to 250 atmospheres, at a con centration of acetylene in the gas phase which is at least Y 10% by volume but below a concentration falling above the decomposition limiting pressure of the gas mixture and at a concentratlon of the acetylene in the liquid phase 5. A method as claimed in claim 4, wherein a solution of nickel acetyl acetonate in dry tetrahydrofurane is saturated with pure acetylene at about 20 C. at a pressure of about 7 atmospheres of it into the reaction zone.

6. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of dry benzene as diluent containing'about 0.6 percent nickel acetyl acetonate as catalyst at a temperature between 105 and 115 C., the improvement which comprises carrying out the reaction in a reaction zone with a mixture of acetylene and nitrogen at a total pressure of 200 atmospheres, said gas mixture containing about 20 percent by volume of acetylene, and introducing a solution of about 0.35 percent of the aluminum enolate of alkyl acetoacetate in benzene saturated at a temperature between 130 and 140 C. and a pressure of about 200 atmospheres with a gas mixture of acetylene and nitrogen containing about 20 percent by volume of acetylene.

7. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of dry benzene as diluent containing about 0.6 percent nickel acetyl acetonate as catalyst at a temperature between 105 and 115 C., the improvement which comprises carrying out the reaction in a reaction zone with a mix- 'ture of acetylene and nitrogen at a'total pressure of 200 atmospheres, said gas mixture containing about 20 percent by volume of acetylene, and introducing into said reaction zone a solution of about 0.35 percent of the aluminum which is below the ignition limiting concentration of the liquid mixture.

4. In a method of producing cyclo-octa-tctraene by cyclizing polymerization of acetylene in the presence of dry tetrahydrofurane as diluent containing about 0.5 to 1 percent nickel acetyl acetonate at a temperature between about and C., the improvement which comprises carrying out the reaction in a reaction zone with a mixture-of acetylene and nitrogen at a total pressure of about atmospheres, said mixture of acetylene and nitrogen containing about 22 percent by volume of acetylene.

enolate'of alkyl acetoacetate in benzene which has been saturated with pure acetylene at a temperature of about 20 C. and a pressure of about 7 atmospheres.

8. In a method of producing cyclo-octa-tetraene by cyclizing polymerization of acetylene in the presence of an inert diluent and a bivalent nickel compound as a catalyst, the improvement which comprises carrying out the reaction at a temperature of about 85 C. to C.

in a reaction zone containinga gaseous mixture of acety-' lene and an inert gas at a total'pressure of from to 250 atmospheres, the concentration of acetylene in the gas phase being between 10% and about 23% by volume.

References Cited in the file of this patent UNITED STATES PATENTS Chemistry, Reinhold Publishing C0,, NW York (1949) pp. 318-321 relied on.

acetylene before introducing V 

